1. Field of the Invention
This invention relates to a driveline that directs power to forward and rear axles, in particular it pertains to a lubrication system for a transmission and transfer case.
2. Description of the Prior Art
A conventional motor vehicle equipped with an automatic transmission should be towed with its drive wheels out of contact with the ground, preferably with the vehicle supported on a tow truck or with the drive wheels supported and the non-driven wheels on the ground. A similar requirement applies also to a vehicle having an automatic transmission and a transfer case. If the vehicle were towed with the drive wheels on the ground, certain transmission and transfer case components would be driven in rotation by reverse torque from the drive wheels. With the engine stopped, the transmission pump cannot supply lubrication fluid to rotating components of the transmission. In order to avoid damage to these components due to lack of lubrication to the rotating components, the distance and speed at which such a vehicle can be towed with the drive wheels in contact with the ground is limited. Frequently there is a need for such vehicles to be towed long distances at highway speed with all the wheels contacting the road surface.
A transfer case usually includes a planetary gear set for producing either a “high” range, in which the transfer case output is driven at the same speed as the input, or a “low” range, in which the output is driven slower than its input speed. The 4×2 and 4×4 states of the transfer case are usually selected or controlled manually by the vehicle operation by operating a lever or switch. At a first position of the selector, the planetary gear set of the transfer case directs power from the transmission output to a front drive axle. At a second position of the selector, the transfer case directs power to both a front drive axle and a rear drive axle, the 4×4 drive mode.
The oil sump of a transfer case may fill with transmission fluid used to lubricate critical rotating components and to operate the hydraulically actuated clutches and brakes. Typically there is no gravity feedback of oil to the transmission sump from the transfer case sump because the transfer case sump is located at a lower elevation than the transmission sump, or because the hydraulic path between them is obstructed to fluid flow.
Yet it is desirable to improve the fuel economy and function of a vehicle equipped with a driveline having a transfer case and an automatic transmission by reducing hydraulic drag losses in the driveline caused by passing rotating components through fluid contained in a transfer case oil sump. A drive mechanism for transmitting power from the output shaft of the transfer case to the forward drive shaft typically rotates with an output sprocket wheel and chain moving in the sump. By operating with the level of oil in the sump at a low level or with a dry sump, the fuel economy of the vehicle can be improved by avoiding hydraulic drag losses.
In order to improve fuel economy it is desirable that the sump of the transfer case be drained continually and its contents be used to supply lubrication fluid to the transmission. Furthermore it is desirable that fluid that collects in the transmission oil sump be supplied to lubricate the transfer case.